I figured it out now! Only a few numbers are produced with the first part (adding digits then taking that away) and those numbers all have the same symbols. Look at the ones you've done - they're the same.

either you did the calcul wrong or you didnt watch the right image. 15 : 1+5 =6 15-6 = 9.
He answer is always a multiple of 9. You can try with any number between 10 to 99 it will always give you a mutiple of 9. 10 : 1+0 = 1 10-1 = 9. 75 : 7+5 = 12 75-12 = 63. 63 is a mutiple of 9. You can look at all the multiple of 9 it will always be the same image.